Preparation of phosphorus oxybromide



June 9, 1964 l.. R. BELoHLAv ETA-L 3,135,605

PREPARATION oF PHosPHoRUs oxYBRoMIDE Filed NOV. 17. 1961 TEMI? I Iwo/c4 rae l l I l Il# 'WE' T 7'56' T ME TER ATTORNEY 3,136,606 f PREPARATION F PHOSPHGRUS OXYBRMIDE Leo Rudolf Belohlav and Conrad Martin Ness, both of Lafayette, Ind., assignors to Great Lakes Chemical-Corporation, West Lafayette, Ind., a corporationV of Michigan Filed Nov. 17, 1961, Ser. No. 153,138

3 Claims. (Cl. 23-203) This invention relates to an improved method forthe preparation of phosphorous oxybromide.

Phosphorus oxybromide is becoming of increased importance as a raw material for the preparation of flameretardant compositions. Although phosphorus oxybromide has been known for a considerable time, it has not been available in large quantities heretofore apparently because no adequate commercial method for its preparation was known. While the most. straightforward approach to the preparationof phosphorus oxybromide would be the direct oxidation of phosphorus tribromide, oxygen and phosphorus tribromide can beheated up to the boiling point of the latter without any apparent reaction taking place. v K

Geuther and Michaelis, Ber., 6, 766 (1871), in 'an attempt to prepare the bromine analog of pyrophosphoryl chloride by reacting gaseous nitrogen trioxide and nitrogen tetroxide with phosphorus bromide,`noted the formation of phosphorus oxybromide and phosphorus pentoxide. This reaction was further pursued by Johnson and Ninn, I. Am. Chem. Soc., 63,142 (1941), who treated samples of phosphorus tribromide with varying amounts of nitric oxide and oxygen in a closed system. -Basing their tindings primarily on the amount of oxygen consumed, the authors proposed the hypothesis that phosphorus-oxybromide is the main product formed in the non-explosive oxidation of phosphorus` tribromide. Using the method of Johnson and Ninn, it vis extremely ditiicult, if not impossible, to'react phosphorus tribromide with oxygen inthe presence of nitrogen dioxide to make practical quantities of phosphorus oxybromide without the danger of explosion.

We have discovered' thatvthe product itself, phosphorus oxybromide, is an excellent moderator for the abovev reacf tion. When phosphorus oxybromide is presentin the initial reaction mixture in an amount at least 30 percent -United States Patent 3,136,606 Patented June 9, 1964 ICC ` Since the process requires intimate contact between the oxygen-rich gas phase and the liquid phase containing phosphorus tribromide, the rate of `reaction is promoted by eicient stirring.. ANitrogen dioxide need be .present in the' reaction mixture only in catalyticv amounts, conveniently 0.1 to"5 volume percent in the gas phase; Neither the concentration of oxygen nor the concentration of nitrogen dioxide is critical; butin order to achieve, prac tical rates of reaction, minimum quantities of 0.1 and-20 volume percent of nitrogen .dioxide and oxygen, respectively, should be-present. As expected, increasing the ratio of these gases in the reaction mixture will increase the rate of reaction observed. But an increase of beyond 5 volume percent of nitrogen dioxide will afford no further practical increase in the rate of reaction.

The Johnson and Ninn paperv citedksupra postulates nitrogen tetroxide to be the oxide of nitrogen kinetically involved'- in catalyzing the reaction of phosphorus tribromid'e and oxygen to yield phosphorusoxybromide. As is well known, nitrogen tetroxide and lnitrogen dioxide exist inequilibrium with each other according to the equation 2NO2N2O4. For convenience, nitrogen dioxide is referred to throughout the specicationas being the catalyst, although it is understood that the tetroxide may be the form-which governs the kinetics of the reac tion. A A

Nitric oxide reacts readily with oxygen to form ,nitrogen dioxide, and it is usually more convenient to add nitric oxide to the reaction mixture forming the nitrogendioxide in situ. V'Since nitrogen sesquioxide (N203) behaves as A laboratory scale run was made using an apparatus consistingA of a ilask Yequipped witha'stirrer and with a gas inlet for nitrogen tetroxide and oxygen, a gas outlet l for exit gases, and measuring devices for the' inletl and exit byv weight of saidmixture, no explosions occur even at extremely high concentrationsof the, catalyst, and; the rate of reaction isreadily controlled by-external cooling or heating of the reaction Vessel.

The invention sought to be patented resides in the concept of a process for the preparationof phosphorus oxybromide which includes introducing phosphorus tribromide, nitrogen dioxide, and an oxygen-containing gas into a heated reaction zone, and maintaining at least 30 percent by weight of phosphorus oxybromide in the reaction mixture in said zone.

The manner and process of using this invention is illustrated by the following general description and examples, which set forth the best mode contemplated by us of carrying out the invention vso as to enable any person skilled inthe art of chemistry to make and use the same: The critical reaction temperature range for the nitro gen dioxide catalyzed reaction of phosphorus tribromide and oxygen is between 60 and 150`degrees centigrade with the preferred range being between 100 and 130 degrees centigrade. While more favorable reaction rates are obtained above 100 degrees centigrade, the temperature should be held below 130 degrees oentigrade to minimize thermal decomposition in the system. Oxygen may be provided as air or other otherwise unreactive gas mixtures containing 20 to 100 volume percent of oxygen, the gas stream being freed from moisture prior to use.

gases. The flask was charged with v1750 cubic centimeters of a mixture'cor'rprised of 23:5 percent b-yweight of phosphorus tribror'nider and 76.5:percent by weightof phosphorus oxybromide. Theilaskwas brought to the reaction'temperature and dry oxygen containing the nitric oxideadded. Oxygen uptake wasldete'rmined by measuring the diiference in rate between the inlet and exit gases. The reaction was run for approximately three I hours at temperatures of Vfrom. to 150 `degrees centigrade; oxygen uptake rates of from 1.2 to 0.1 liters per minute were measured during that time. After the oxidation was complete, as indicated by the decrease in oxygen uptake, a sample of the reaction mixture was withdrawn from the flask and found to contain 97.5 percent of phos- Subsequent vacuum distillation of this material in a single stage distillation apparatus pro duced'a colorless product whose properties match those given in the literature for pure phosphorus oxybromide.

Example 2 A pilot scale run was made in an apparatus similar to that illustrated in FIGURE 1,. The apparatus consisted of a spray type reaction chamber, a surge pot, a pump, and a heat exchanger along with the accessory equipment required for the addition and measurement of the gas and liquid `reactants and measurement of the exit gases. The system was charged with approximately 3000 cubic centiu meters of a mixture comprised of 26 weight percent of phosphorus tribromide and 74 Weight percent of phosphorous oxybromide. The liquid reactants were recirculated into the reactor througha spray nozzle at rate of approximately 80() cubic centimeters per minute. After the temperature of the system was brought kup to the desired range, the addition `of oxygen and nitric oxide was begun. The reaction .progressed for 11/2 hours at temperatures ranging from 120 to 130 degrees centigrade and at oxygen uptake rates up to 3 liters per minute. During this time a The process can also vbe run on a continuous basis by the use of multiple reactors in series, or using a single reactor with multiple passesin series.

-theoxygen concentrationY in the reaction chamber was Example 3 Y The semicontinuous operation of our process utilizing the apparatus shown in FIGURE 1 is as follows:- A mixture containingV the prestated proportions of phosphorus oxybromide and phosphorus tribromide is placed in a surge pot 10. Apump 12 draws the mixture through a filter 11 and passes it through a heater 13. The material then is sprayed4 into a reactor 16 througha spray nozzle 15. From the reactor it drops through a trap and back to the surge pot 10.` The oxygen and nitric'oxide Vare passed through their respective meters 17 and 18 and driers 19 and 20 and into the reactor Y16. The olf-gas line from the reactor 16 is provided with a trap 21, a drier `22,.and a gasometer 23. At the start of an operating cycle, the heater 13 Visused to bring the temperature in the system upto Vthe point where the 'oxidation reaction commences. Measurement of the temperature within the reactor 16 gives an accurate indication of the reaction rate. As the reaction rateV increases thearnoun't of heat supplied by the heater 13 can be decreased due to the 1 exothermic nature of the oxidation. The reaction rate can also be followed by the rate of oxygenconsumption.

As the oxidation of the liquid charge nears completion, the rate of oxygen consumption decreases as shown by its increasing appearance in the oli-gas stream. VAt the same time, the reactor temperature falls off. The melting point of the liquid/is then checked to determine the phosphorusfoxybromide content@ When the desired concen- The subjectmatter which'the applicants regard as their Yinvention is particularly pointed out and distinctly claimed as follows; Y 1. In a process for the preparation of phosphorus oxybrornide (POBr3)` by the oxidation with uncombined oxygen in its molecular form of phosphorus tribromide at a temperature of about 60 to 150 degrees centigrade, catalyzed with at least 0.1 volume percent of nitrogen dioxide (NO2), the improvement which comprises maintaining in the reaction mixttn'e at all times during the oxidation at least 30 percent'by weight of phosphorus oxybromide (POBr3) calculated on the reaction mixture.

2. In afprocess'rfor the preparation of phosphorus oxy- .bromide (POBr3) by the oxidation with.V uncombined oxygen in its molecular form of phosphorus tribromide at a temperature of about 60 to 150 degrees centigrade, `catalyzed with at least 0.1 volume ypercent of nitrogen dioxide (NO2), the improvement which comprises conducting the oxidationby spraying a mixture of phosphorus tribromide. and atall timesfat leastv 30 percent by weight of phosphorus oxybromide into a gaseous atmosphere containing the oxygen and the nitrogen dioxide until oxygen uptakeubstantially ceases. Y i

3. In a process for the preparation of phosphorus oxybromide (POBrg) by the oxidation with uncombined koxy- 1 Vgen in its molecular form ofy phosphorus tribromide'ata temperature of about 100 to 130 degrees centigrade, catalyzed with at least, 0.1 volume percent of nitrogen dioxide (NO2), the improvement whichfcomprises thor',- oughly mixing a'liquid mixture ot'` phosphorus tribromide and at least 30 percent by` weight of phosphorus oxybromide with the oxygen and the nitrogen dioxide, re-

moving phosphorus oxybrornideV from the reaction mixtration is reached, a lportion of the liquid is 'withdrawn from the bottom of the reaction` 16, more phosphorus tribromide is added yto the system yia the surge pot 10, and the cyclevis repeated; The concentration ofl phosphorus oxybrornide in the reactor is never allowed to fall below 30 percent at any time.

and 1036..y

ture and adding additional phosphorus tribromide thereto, while at all ,times maintaining in the reaction mixture at j least 30 percent lby weight of phosphorus oxybrornide.`

References'Cted in the iilerof this patent UNTED STATES PATENTS Britton et al Nov. 22, 1932 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol.- 8, Longmans,V Green and C0., N.'Y'., 1928, pages 382, 1002, 1012, 10211032, 1035 l Johnson eta al.: Catalytic .Oxidation ofr Phosphorus Tribromide, Journal of the American Chemical Society,

Y vol. 63,- January-Iune. 19'41, pages 141-143. 

1. IN A PROCESS FOR THE PREPARATION OF PHOSPHORUS OXYBROMIDE (POBR3) BY THE OXIDATION WITH UNCOMBINED OXYGEN IN ITS MOLECULAR FORM OF PHOSPHORUS TRIBROMIDE AT A TEMPERATURE OF ABOUT 60 TO 150 DEGREES CENTIGRADE, CATALYZED WITH AT LEAST 0.1 VOLUME PERCENT OF NITROGEN DIOXIDE (NO2), THE IMPROVEMENT WHICH COMPRISES MAINTAINING IN THE REACTION MIXTURE AT ALL TIMES DURING THE OXIDATION AT LEAST 30 PERCENT BY WEIGHT OF PHOSPHORUS OXYBROMIDE (POBR3) CALCULATED ON THE REACTION MIXTURE. 